Effects Of Litter And Understory Vegetation Removal On Soil Nitrogen Mineralization In Forest Ecosystems In Different Climatic Zones Of China

Forest ecosystems are a key component of the productivity of terrestrial vegetation.Plant growth need most of the nutrients,such as C,N,P,etc.,which are supplied by decomposition of the litter and soil organic matter.Available nitrogen is one of the main factors limiting the forest ecosystems productivity.As a result,studying the mineralization of nitrogen is important for understanding nutrient cycling process and transformation balance of forest ecosystems,and has important guiding value and practical significance for understanding the productivity of forest ecosystem.Surface litter and understory vegetation may affect soil mineralization and soil nitrogen balance by changing soil physicochemical properties and soil microbial activities.At present,however,there are few controlled experiments associated with litter or understory vegetation removal was conducted to study soil nitrogen cycling in different climatic zones.In this study,four treatments,Contrast Check,Litter Removal,Understory Removal,Litter and Understory Removal were applied in three forest vegetation type,Dinghu Mountain in the subtropical zone,the Baotianman in the warm temperate zone and the Changbai Mountain in the middle temperate zone.Four treatments were we used in-situ nitrogen mineralization culture experiment by resin core method to explore the following scientific problems:(1)The effect of litter removal and understory vegetation removal on soil physicochemical properties,microbial biomass and nitrogen mineralization rate;(2)The response mechanism of soil nitrogen transformation process to litter and understory removal in different climatic zones.Our study will provide data support and theoretical basis for forest management in different climatic zones.In view of the above scientific problems,the soil physical and chemical properties,microbial biomass carbon and nitrogen,inorganic nitrogen content and nitrogen mineralization rate were determined.We reached the following conclusions:(1)Litter removal and understory vegetation removal affected soil nitrogen mineralization rate by changing soil physical and chemical properties and microbial biomass.Litter removal significantly increased soil net nitrification and net mineralization rate,which may be caused by reduction in soil water content,and increase in soil temperature and SOC,leading to increased soil permeability and oxygen content which associated with higher nitrification of nitrifying bacteria;UR treatment significantly reduces the soil net nitrification rate,probably because of reduced soil pH under UR,and the acidic environment is not conducive to the nitrification of microorganisms.The soil nitrogen mineralization rate under different forest vegetation types have different responses to treatment of litter removal and understory vegetation removal.(2)For the subtropical Dinghu mountain,litter quantity is small,but the understory vegetation is dense.LR significantly increases the rate of soil mineralization which may be caused by higher SOC.UR significantly reduced the soil net ammoniation and net mineralization rates because understory removal inhibits the photosynthetic product transported to the soil,which further lead to inhibiting of the microbial life activities.(3)In the case of Baotianman in the transition zone between subtropical and mid-temperate zones,the net nitrification and net mineralization rates under LR treatment showed a significant increase at the first and second sampling,and then a significant reduction after the third sampling.Furthermore,LUR treatment significantly reduced the net nitrification rate of soil,indicating that the net mineralization rate is more sensitive to the response of litter removal.With the advancement of litter removal treatment,the soil water content and soil microbial biomass decreased significantly,which may result in the inhibition of mineralization of soil microbes.(4)For Changbai Mountain in the middle temperate zone,LR treatment significantly increased the net nitrification and net mineralization rate,but UR treatment had no significant main effect,indicating that the soil mineralization rate is mainly due to the higher litter quantity in the Changbai Mountain.The litter layer in the Changbai Mountain plot is the thickest,but the number of understory vegetation is the least.After removing the litter,the increased soil net mineralization may be caused by higher soil temperature.